Deposition of aluminum on a galvanized surface

ABSTRACT

A galvanized substrate is plated with aluminum metal by initially heating the substrate to within a temperature range below the melting point of zinc, contacting the heated substrate with a plating solution containing an aluminum alkyl compound: thereby, depositing a thin coating of aluminum on the substrate, raising the temperature of the coated substrate to within a range above the melting point of zinc and contacting the coated substrate with the plating solution, achieving an aluminum plating of a desired thickness on the galvanized substrate.

United States Patent Hood 1 51 Mar. 28, 1972 [54] DEPOSITION F ALUMINUM ON A 3,449,144 6/1969 Williams et al ..117/130 R x GALVANIZED SURFACE 3,449,150 6/1969 Williams et al. ..117/50 3,549,412 12/1970 Frey, Jr. et a1. ...1 17/130 R X 1 lnvemofl Hood, Ponca Y Okla- 3,578,494 /1971 Williams et a1 ..117/130 R x [73] Assignee. gtlllllgtnental Oil Company, Ponca City, Primary Examiner Alfred L Leavitt Assistant Examiner-J. R. Batten, Jr. Filedi 7, 1970 Attorney-Joseph C. Kotarski, Henry H. Huth, Robert B. [21 1 App]. No: 64,626 Coleman, Jr., Ronnie D. Wilson and Carroll Palmer [57] ABSTRACT [52] U.S. C1 ..ll7lfliaf79ffi6ffi/alggk A galvanized Substrate is plated i aluminum metal y 51 1111. c1. ..B32b /18 C236 3/00 B32b 15/20 heating the F range 58 Field of Search ..117/71M 130 R 29/1965 the melting macmg the 29/197 strate with a plating solution containing an aluminum alkyl compound: thereby, depositing a thin coating of aluminum on the substrate, raisin the tem stature of the coated substrate [56] References Cited to within a range abive the m zlting point of zinc and contact- UNITED STATES PATENTS ing the coated substrate with the plating solution, achieving an aluminum plating of a desired thickness on the galvanized sub- 3,041,l97 6/1962 Berger ..117/ R X Straw. 3,214,288 10/1965 McGraw.... ....1 17/130 R X 3,438,754 4/1969 Shepard et a] ..117/71 M X 7 Claims, No Drawings DEPOSITION OlF ALUMINUM ON A GALVANIZED SURFACE This invention relates to a method for chemically plating aluminum metal on a galvanized substrate utilizing a heat decomposable aluminum alkylcompound in a liquid plating solution.

PRIOR ART Various methods have been reported disclosing the coating of both metallic and nonmetallic substrates with aluminum metal. A well-known method is described in Berger U.S. Pat. No. 3,04 l l 97. Berger discloses the plating of metal substrates with aluminum by contacting the metal with an aluminum alkyl compound at a temperature at which the aluminum alkyl compound undergoes decomposition. However, Berger nor any of the other previous methods are addressed to the application of a thick coating of aluminum on a galvanized substrate and the problems appurtenant thereto.

Previously, an aluminum coating on a metal substrate of any substantial thickness required a repetition of the steps of heating, contacting with a suitable plating solution, and rinsing with a solvent to achieve an aluminum coating of only questionable quality. Further, thick coatings of aluminum metal on a galvanized substrate have been achieved by electrolytic plating, but this method is expensive, time consuming and the application is difficult to control. Many of the prior methods have required extensive surface preparation prior to plating the aluminum metal on the galvanized substrate.

In order to obtain a useful and quality coating of aluminum on a galvanized substrate, it is of primary importance to utilize temperatures which are high enough to cause thermal decomposition of the plating solution yet low enough so as not to cause the galvanize to melt and flow on the surface of the substrate. The thickness of an aluminum plate deposited on a galvanized substrate is dependent on the amount of time that the galvanized substrate is in contact with the aluminum alkyl solution at the requisite temperature to achieve plating. This amount of contact time can otherwise be denoted as the plating period. The length of this so-called plating period is determined by the temperature of the substrate and the temperature of the plating solution at the time of immersion of the substrate in the plating solution. Consequently, the longer the conditions of plating are maintained, thereby lengthening the plating period, the thicker the subsequently achieved aluminum plate on the substrate will be. Therefore, the thickness of the aluminum coating deposited on the galvanized substrate increases in direct proportion to the increase in time of the plating period. I

It is the object of the present invention to provide a method for preparing a galvanized substrate having a thick coating of aluminum metal. More specifically, the object of the invention is to plate aluminum metal on a galvanized substrate from a solution of one or more heat decomposable aluminum alkyl compounds. The further object of the invention is to provide a method whereby a galvanized substrate can be prepared having an aluminum coating of a desired thickness.

A galvanized substrate having an aluminum metal coating deposited thereon by the method of the present invention affords the metal so coated a much higher degree of corrosion protection than a metal surface coated only with zinc. Therefore, a galvanized substrate having a thick coating of aluminum thereon would be useful as parts in the exhaust systems of automobiles, trucks, buses, etc., for example, as tail pipes and mufflers. These and other objects and advantages of this invention will become apparent upon reading the following description.

It has now been discovered that upon heating a galvanized substrate to a temperature within a range below melting point of the galvanize, contacting the heated substrate with an aluminum plating solution to deposit a thin coating of aluminum metal on the substrate; and reheating the so-coated substrate to a temperature within a range above the melting point of the galvanize, then, contacting the reheated-coated substrate with the aluminum plating solution so that a second layer of aluminum metal is deposited on the aluminum-galvanize coated substrate; thereby, obtaining a galvanized substrate having a thick plate of aluminum metal adhered thereto.

An aluminum plating solution which may be employed in the present invention is a liquid solution of one or more of the types referred to in Berger, U.S. Pat. No. 3,04l,l97. More specifically, the aluminum plating solution employed in the present invention is a solution of one or more aluminum alkyl compounds having the structural formula AIR(R), where R is an alkyl radical containing from about 1 to about 30 carbon atoms and R is selected from the group consisting of alkyl radicals containing from 1 to about 30 carbon atoms and hydrogen. Also, a pure solution of aluminum alkyl compounds may be employed in the process described herein or a suitable solvent can be utilized with the aluminum alkyl compounds in the plating solution. A suitable solvent for the aluminum alkyl compounds is'a mixture of liquid hydrocarbons which are inert with respect to the aluminum alkyl compound and also with respect to the substrate material in use. The hydrocarbon solvent to be employed is selected to have a boiling point which is about 10 C. to 50 C. below that of the aluminum alkyl compound utilized. Typically, a kerosene cut which meets the aforementioned requirements is employed as a solvent in the plating solution. A representative solution employed will contain about 50 weight percent aluminum alkyl compound and 50 weight percent solvent.

After the initial thin coating of aluminum metal has been deposited on the galvanized substrate, a rinsing step may be employed in the present invention, but it is not essential to achieve a thick coating of aluminum metal. If the rinsing step is employed in the present invention, a suitable solvent should be employed which is nonreactive with the galvanized substrate or with the plated aluminum. A typical example of a suitable solvent would be kerosene or a similar hydrocarbon cut. The rinsing step would be employed for the purpose of rinsing off excess alkyl that did not plate on the galvanized substrate because of the termination of the previously mentioned plating period.

Careful temperature control must be maintained in aluminum plating of a galvanized substrate by contact with an alkyl aluminum solution. The mechanics of the plating process involve the thermal decomposition of the alkyl aluminum compound to deposit metallic aluminum on the surface of the galvanized substrate. The temperature at which such decomposition will occur varies with the type of aluminum alkyl compound used in the plating solution. Further, the temperature at which the galvanize will melt and run on the surface of the substrate is of critical importance in the present invention. It is imperative that at the first deposition of aluminum metal on the galvanized substrate that the substrate not be heated above the melting point of zinc so as to avoid the problems mentioned above. Therefore, at the time of initially heating the galvanized substrate, a temperature below the melting point of zinc must be employed in the present invention. Further, in selecting a suitable aluminum alkyl compound for the plating solution such a compound should be chosen which has a decomposition temperature which is below the melting point of zinc. Consequently, the initial heating of the galvanized substrate should be at a temperature from about 275 C. to about 415 C. Preferably the temperature employed should be in a range from about 375 C. to about 400 C.

After the initial thin coat of aluminum metal has been deposited on the galvanized substrate, it has been found that the critical limit of the melting point of zinc no longer applies. If a galvanized substrate were heated above the melting point of zinc and immersed in a coating bath, it would be expected that the zinc coating would melt and run into the solution. Further, it would not be expected that a thin layer of aluminum metal having a thickness of from about 0.001 mils to about 0.05 mils would offer sufficient insulation for the zinc coating on the surface of the substrate to prevent the zinc from melting and running at temperatures above the melting point ofzinc.

By the method of the present invention, it has surprisingly been found that the initial thin coating of aluminum on the galvanized substrate affords such protection to the zinc coating that when temperatures above the melting point of zinc are utilized on the substrate the previously mentioned problems do not occur, thereby, allowing the utilization of higher temperatures to achieve thicker aluminum plating. Therefore, upon reheating the aluminum galvanize coated substrate, a temperature higher than the melting point of zinc can be utilized to obtain a thick aluminum plating thereon. Consequently, the temperatures utilized upon reheating the aluminum zinc-coated substrate should be in the range of from about 420 C. to about 500 C. As a preferred embodiment the aluminum zinc-coated substrate should be heated to a temperature in the range of about 430 C. to about 460 C.

Although decomposition of an aluminum alkyl compound present in a plating solution will occur on contact with the galvanized substrate, without prior heating of the plating solution, it is a preferred embodiment of the present invention to heat the plating solution to a temperature from about 100 C. to about 175 C. prior to contacting the substrate therewith. The preheating of the aluminum alkyl plating solution lessens the temperature differential between the galvanized substrate and the plating solution thereby lengthening the time of the so-called plating period. The smaller the A t between the temperature of the substrate and the temperature of the plating solution the slower will be the temperature drop of the substrate upon contacting it with the plating solution. Thus, the conditions for good plating are maintained for a relatively longer period of time than that achieved if the solution is at room temperature, to allow a thick coating of aluminum to be deposited on the galvanized substrate.

The higher temperatures utilized in the second coating step allow a thick coating of aluminum to be deposited on the galvanized substrate without the problems of the zinc coating melting and flowing on the surface ofthe substrate.

A desired thickness of an aluminum metal on a galvanized substrate can be achieved by the present process by varying the temperature of the substrate and the temperature of the plating solution thereby controlling to a substantial degree the length of the plating period.

At the initial contact of substrate with coating solution in the present solution, coatings of aluminum ranging from a thickness of about 0.001 mils to about 0.05 mils can be achieved. It should be kept in mind that this initial contact of the substrate with plating solution should take place at a temperature below the melting point ofzinc.

In the second contacting step the coatings of aluminum ranging from a thickness of about 0.001 mils to about 0.25 mils may be obtained. Further, it should also be remembered that one of the primary requirements of achieving a thick aluminum coating on a substrate in this second step, is that on contacting with the plating solution the substrate is at a temperature in the range above the melting point of zinc. It should be noted that to achieve even thicker coatings of aluminum on a galvanized substrate, when requirements so dictate, that repeated applications of aluminum can be utilized to give greater thicknesses under the conditions as described herein as the second coating step.

Method of the present invention may be carried out with any suitable apparatus which provides for heating the galvanized substrate and for contacting the heated substrate with a plating solution, such as those described in co-pending U.S. Pat. application Ser. No. 815,690 filed Apr. 14, 1969, and Ser.

method of the present invention.

EXAMPLE 1 A galvanized steel pipe 2.5 inches in length and 1.0 inches in outside diameter was heated to 393 C. and immersed in a plating solution of solvent-free diethyl aluminum hydride having a temperature of C: The pipe was subsequently rinsed in kerosene and reheated to 427 C. and again immersed in the plating solution. A coating of aluminum metal 0.16 mils in thickness was deposited on the galvanized steel pipe.

EXAMPLE 2 Performing the same steps and utilizing like materials as referred to in Example 1, a galvanized steel pipe was coated with an aluminum metal coating 0.16 mils thick. The pipe was initially heated to 393 C. and subsequently heated to 422 C. The plating solution of solvent free diethyl-aluminum hydride had a temperature of 158 C. at the times of immersion.

The invention having thus been described, I claim:

1. A method for depositing a thick aluminum metal coating on a galvanized substrate with a plating solution containing an aluminum alkyl compound which comprises the steps of: heating said substrate to a temperature below the melting point of the galvanize, contacting said heated substrate with said plating solution to deposit a thin coating of aluminum metal thereon, heating the thinly coated substrate to a temperature above the melting point of the galvanize, and contacting said heated, thinly coated substrate with said plating solution to deposit a second coating of aluminum metal on the aluminumgalvanize coated substrate, thereby obtaining a galvanized substrate having a thick plate of aluminum metal adhered thereto.

2. The method of claim 1 wherein said plating solution contains an aluminum alkyl compound having the structural formula of AlR(R) wherein R is an alkyl radical containing from about 1 to 30 carbon atoms and R is selected from a group consisting of alkyl radicals containing from l to about 30 carbon atoms and hydrogen.

3. The method of claim 2 wherein said plating solution is heated to a temperature below the decomposition point of the aluminum alkyl present in said plating solution prior to contacting with said heated, thinly-coated substrate.

4. The method of claim 3 wherein said plating solution is heated to a temperature from about 100 C. to about C.

5. The method of claim 2 wherein said substrate is initially heated to a temperature of from about 275 C. to about 415 C. and said thinly-coated substrate is heated to a temperature offrom about 420 C. to about 500 C.

6. The method of claim 5 wherein said substrate is initially heated to a temperature of from about 375 C. to about 400 C. and said thinly-coated substrate is heated to a temperature offrom about 430 C. to about 460 C.

7. The method of claim 5 wherein said thin coating of aluminum metal on the galvanized substrate has a thickness of from about 0.001 mils to about 0.05 mils and said second coating of aluminum metal on the aluminum galvanize coated substrate has a thickness of from about 0.001 mils to about 0.25 mils. 

2. The method of claim 1 wherein said plating solution Contains an aluminum alkyl compound having the structural formula of AlR(R'')2 wherein R is an alkyl radical containing from about 1 to 30 carbon atoms and R'' is selected from a group consisting of alkyl radicals containing from 1 to about 30 carbon atoms and hydrogen.
 3. The method of claim 2 wherein said plating solution is heated to a temperature below the decomposition point of the aluminum alkyl present in said plating solution prior to contacting with said heated, thinly-coated substrate.
 4. The method of claim 3 wherein said plating solution is heated to a temperature from about 100* C. to about 175* C.
 5. The method of claim 2 wherein said substrate is initially heated to a temperature of from about 275* C. to about 415* C. and said thinly-coated substrate is heated to a temperature of from about 420* C. to about 500* C.
 6. The method of claim 5 wherein said substrate is initially heated to a temperature of from about 375* C. to about 400* C. and said thinly-coated substrate is heated to a temperature of from about 430* C. to about 460* C.
 7. The method of claim 5 wherein said thin coating of aluminum metal on the galvanized substrate has a thickness of from about 0.001 mils to about 0.05 mils and said second coating of aluminum metal on the aluminum galvanize coated substrate has a thickness of from about 0.001 mils to about 0.25 mils. 